584 Hill. — The Histology of the 
batmen.’ Protoplasmic threads become changed into callus- 
rods, but the refractive nodules are not situated on these rods 
as formerly supposed, for they are now found to alternate with 
the callus-rods and to represent the mesh work of the middle 
lamella between the callus, and this meshwork remains after 
the solution of the callus. These conclusions were arrived at 
by the help of thinner sections and greater magnification than 
had been formerly employed. His experiments have been 
repeated— as have those of Russow, &c. — but, as will be seen 
later, my results do not at all agree with these latest views 
put forward by Strasburger. 
In the paper on ‘The connecting threads in Pinus sylves- 
tris 1 5 some figures of sieve-tube ‘ threads ’ are given : these, 
except in Fig. 29, are in reality the slime-strings with median 
nodes ; the grouping of the slime-strings also is not well shown, 
and no callus is indicated. These discrepancies are due to the 
employment of the acid-violet method, which is not the best 
method for a research into the structure of the sieve-plate. 
An important memoir was published by Perrot 1 2 in 1899 
on sieve-tubes in general, which on the whole is a very useful 
rhsumhoi previous research. His account of the structure of the 
sieves of Gymnosperms is, however, very imperfect and Rus- 
sow’s valuable work receives but scant attention at his hands. 
To summarize then the most important views which are 
held concerning the structure of the sieve-plate : — Two distinct 
sets of opinions have been published, firstly, those of Russow 
with which Strasburger’s 1884 account mainly agrees ; and 
secondly, those given by Strasburger in his later works 3 . 
Russow found that in the mature condition the sieve-plate 
is traversed by groups of slime-strings enclosed by callus- 
rods, and that at the middle lamella each slime-string bears 
a nodule or swelling, which disappears on the solution of the 
callus. 
The callus, which is formed by deposition from the proto- 
1 Hill, loc. cit., Figs. 26-29. 
2 Perrot, * Le tissu crible,’ 1899, pp. 43-47 and 67-72, &c. 
3 * Leitungsbahnen,’ and Jahrb. f. wiss. Bot., 1901. 
